Power transfer systems employing electrical coupling methods are known. In these power transfer system, active electrodes of a power transmission device and a power reception device are located in proximity to each other via a gap, and passive electrodes of the power transmission device and the power reception device are located in proximity to each other via a gap, so that large capacitances are formed between the electrodes, and the electrodes are electrically coupled to each other. Accordingly, electric power transfer between the power transmission device and the power reception device is enabled with high transfer efficiency even if the electrodes of the power transmission device and the power reception device are not in contact with each other.
Examples of the power reception device of the power transfer system include electronic devices such as a cellular phone. In recent years, an electrostatic capacity type input unit (touch panel) having good operability is used in these electronic devices and the like in many cases. In the case where a power reception device is equipped with a touch panel, a situation is possible in which the touch panel is operated while the power reception device is placed on the power transmission device to be charged. In this case, a reference potential at the power reception device side varies, so that the power reception device may malfunction.
Patent Document 1 discloses a power transfer system that stabilizes a reference potential of a power reception device by connecting the reference potential of the power reception device to a power transmission device having relatively small variation of a reference potential. In the power transfer system disclosed in Patent Document 1, each of the power transmission device and the power reception device has a reference potential electrode connected to the reference potential, and the reference potential electrodes are opposed to each other. Thus, the reference potential of the power reception device is connected to the reference potential of the power transmission device, so that the potential of the power reception device is stabilized and operation of the power reception device is stabilized.
Patent Document 1: International Publication No. 2013/054800.
Meanwhile, when the power reception device is placed on the power transmission device, for example, a capacitance (hereinafter, referred to as “cross capacitance”) may be formed between different types of electrodes, for example, between the active electrode of the power transmission device and the passive electrode of the power reception device. The cross capacitance increases particularly if a displacement occurs when the power reception device is placed on the power transmission device. In Patent Document 1, the influence of such a cross capacitance is not taken into consideration, and the reference potential at the power reception device side varies due to the cross capacitance, so that the power reception device may malfunction.